1. Field of the Invention
The present invention relates to an inkjet printing apparatus, and specifically, relates to a configuration for adjusting a distance between a print head and a printing medium such as a printing sheet.
2. Description of the Related Art
In recent years, comparatively inexpensive office automation equipment, such as personal computers and word processors, are used generally; and accompanied with that, various printing apparatuses for printing information input by the office automation equipment, and technologies for increasing printing speed-up and technologies for achieving high printing quality have been developed rapidly. Among the printing apparatuses, a serial printer using dot matrix printing system receives attention as an apparatus for realizing high speed and high printing quality with low cost. In addition, as a technology for performing printing with high quality, for instance, a multi-pass printing system is known.
A high-speed printing can be realized by increasing the number of ink ejection ports in a print head provided with a plurality of the ink ejection ports, and by increasing a scanning speed of the print head. However, a bidirectional printing system is effective as a system for realizing the high-speed printing without such a constraint of the apparatus configuration. A one-directional printing is performed only by a scanning of one-direction movement from a predetermined scanning start position and is accompanied by non-printing movement in a reverse direction to return to the scanning start position from a scanning end position. Therefore, the bidirectional printing can perform the printing at the speed approximately twice as fast as that of the one-directional printing.
The multi-pass printing system as the technology for achieving high image quality can reduce uneven density depending on the variation in an ejection amount and/or an ejection direction of the ink caused by the variation of printing elements such as a shape of the ejection port, an ejection heater performance in the print head. In an ideal print head, the ink is ejected with an approximately uniform ejection amount and in the same alignment direction from the respective ejection ports. Then, when such ejection is performed, the ink dots uniform in size are formed with uniform arrangement on the printing medium, and thus obtaining the uniform image without uneven density as a whole. However, variation in manufacturing the print head or the like causes variation in ejection amount or the like. As a result, there exists a periodical blank part of a printing medium in the dot formation, or inversely, there occurs uneven density where the dots are formed in an overlapping state beyond necessity. In response to this, the multi-pass system carries out plural number of scans to the same printing area, and carries out printing by using different ejection ports in the respective scan. With this configuration, the variation of ejection amount in each ejection port can affect the printing in a state that the variation is dispersed among the plural number of scans, and accordingly, uneven density can be made inconspicuous. In the multi-pass printing, for instance, in the case of two-pass printing where the printing is completed by two scans, the printing data for first and second scans are generated by using masks, and are complemented to each other.
As other example of a technology for achieving high image quality in the dot matrix printing system, there is known a dot alignment technology adjusting a landing position of the ejected ink. The dot alignment is an adjusting method for adjusting a position where the dot is formed on the printing medium by some kind of means. By this adjustment, it is possible to suppress the uneven density caused by deviation of a dot formation position.
For instance, in the method described in the Japanese Patent Laid-Open No. 10-329381, a plurality of patterns are printed by backward and forward scans of the print head, in such a manner that printing start (ejection) timing of the backward scan is shifted by a specific amount to the forward scan for each pattern. These patterns are ones in which an area factor by the dot formed by the printing (ratio occupied by the ink dot in the specific area) differs for every pattern. Then, average density of each of the plurality of patterns is optically read. By this operation, the printing start timing corresponding to the pattern with the highest average density read can be set as a print positioning condition.
However, in the landing position adjustment using the ejection timing described above, it is comparatively difficult to deal with the case where an amount of landing position deviation differs in a conveying direction of the printing medium.
For example, a distance between the print head and the printing medium (hereinafter, also referred to as head-to-medium distance) may be different between an upstream side and a downstream side of the conveying direction of the printing medium. For instance, in some cases, small variation in dimensional tolerance of individual parts such as a platen, a carriage shaft or a chassis rail is stacked and appears as a difference of the head-to-medium distance between the upstream side and the downstream side. In this case, an amount of landing position deviation of the ink ejected from the ejection port at the upstream side in the print head becomes different from that at the downstream side in the print head. As a result, the unevenness of density is caused to be generated by deviation of a dot formation position in the conveying direction.
FIGS. 1 and 2 are diagrams for explaining this problem. FIG. 1 shows a landing position deviation when performing the bidirectional printing with respect to two head-to-medium distances which are different from each other. Here, a print head 11 scans back and forth with a velocity of Vd, and the ink is ejected with a velocity of Vh. As shown in FIG. 1, when the head-to-medium distance is a distance corresponding to “printing medium position 1”, a distance between inks landing in the forward scan and the backward scan of the bidirectional printing is Δxu1. In this case, when the head-to-medium distance changes to a distance corresponding to “printing medium position 2”, the distance between the landing inks at the forward and backward scans changes to Δxl1. In the case where a difference between the printing medium position 1 and the printing medium position 2 is approximately 0.2 mm, the difference between the distance of Δxl1 and the distance of Δxu1 is approximately 20 μm.
As mentioned above, in the case where the distance between the print head and the printing medium is different between the upstream side and the downstream side in the printing medium conveying direction, when the forward and backward printing is performed, in each of the forward and backward scans, the distance between landing inks from the upstream side ejection ports is different from the distance between the landing inks from the downstream side ejection ports. As a result, the unevenness of density is caused to be generated by deviation of dots formed by the landing inks.
FIG. 2 is a diagram explaining that, when an ink (hereinafter, referred to as satellite) is ejected with different velocity accompanied with the main ink droplet, in addition to the main droplet of the ink at the time of ink ejection, the landing position deviation between the main ink droplet and the satellite differs depending on the printing medium position (the head-to-medium distance).
In FIG. 2, the print head 11 scans with velocity of Vc. Then, the main ink droplet is ejected with the velocity of Vh, and the satellite is ejected with the velocity of Vs. In this case, when the printing medium is in the printing medium position 1, the distance of the landing positions between the main droplet and the satellite is Δxu2. When the printing medium is in the printing medium position 2, the distance of the landing positions between the main droplet and the satellite is Δxl2. Then, the distance Δxu2 differs from the distance Δxl2. The main ink droplet is made smaller as a printing resolution becomes higher, and accordingly, a difference of diameter between the satellite and the main ink droplet becomes smaller. Thereby, the satellite is also increasingly apt to add influence to a printing density.
As mentioned above, when the distance between the print head and the printing medium is different between the upstream side and the downstream side in the printing medium conveying direction, the unevenness of density due to the landing position deviation is caused to occur also by the fact that the distance between the landing positions of the main droplet and the satellite of the ink each ejected is different between the upstream side and the downstream side.
In order to suppress occurrence of problems mentioned above, it is desirable that the distance between the print head and the printing medium in the printing medium conveying direction be set to be equal anywhere. For instance, in the case where the landing position deviation caused by a difference of the head-to-medium distance in the printing medium conveying direction is prevented from occurring by using the method for adjusting the ejection timing described above, the ejection timing needs to be different in accordance with the head-to-medium distance, for instance, between the upstream side and the downstream side of an ejection port arrangement. However, in this case, distances between the landing positions of the main droplet and the satellite can not be changed. In addition, as other method, there is known a method in which, at a predetermined position of the printing apparatus, a distance between a member instead of the printing medium and the print head is adjusted, and thereby such adjustment is substituted for the adjustment of the head-to-medium distance. However, such adjusting method does not necessarily reflect actual distance between the print head and the printing medium, and accordingly, in some cases, the unevenness of density caused by the landing position deviation can not be eliminated, as a result.